Class switch recombination (CSR) is an important mechanism for diversification of the humoral immune response. It is initiated by activation induced cytidine deaminase (AID), an enzyme that converts cytidine to uracyl in single stranded DNA thereby creating U:G mismatches that are recognized and processed by mismatch repair enzymes or Uracyl DNA glycosylase (UNG) to produce double strand DNA (dsDNA) breaks. CSR is a deletional recombination reaction that requires formation of paired DSBs that can be separated by as much as 200 kbps in the antibody gene. These breaks must be synapsed and re-ligated efficiently to avoid genomic instability and to create an antibody gene that directs to synthesis of secondary isotypes. During the last funding period we found that the dsDNA break created by AID activates the DNA damage response, which involves recruitment of factors involved in dsDNA break detection, and signaling. We find that in addition to their signaling function these factors are also essential for normal CSR, because in their absence there is preferential loss of the long distance joining between switch regions. In contrast, resolution of closely apposed intra-switch region DSBs proceeds normally even in the absence of dsDNA break detection, and signaling factors suggesting that these factors might be involved in dsDNA break synapsis. An important additional consequence of aberrant repair in the absence of factors such as Nbs1/Mre11Rad50 that detect the break, or the Ataxia Telangectasia Mutated kinase (ATM) which signals to the nucleus chromosome translocation. Among DNA repair factors the most severe defect in CSR is found in the absence of 53BP1, however there is little mechanistic understanding of how DSBs at a distance are synapsed during CSR or how 53BP1 might be involved in this process. The focus of the proposed research is to examine the role of distance between paired DSBs on the same chromosome in repair and how 53BP1 contributes to repair of widely separated breaks during CSR. The long-term goal of the proposal is to understand the molecular mechanisms that regulate CSR and how abnormalities in CSR lead to cancer causing chromosome translocations. PUBLIC HEALTH RELEVANCE: The research proposed is directed at clarifying mechanisms of antibody diversification a process that is essential to all immune serological responses to pathogens. It also seeks to uncover the mechanisms involved in cancer producing translocations.